The present invention relates to cellular radio communications, and more particularly, to downlink radio transmission where multiple antennas are employed at the radio base station.
Shared downlink radio channels, like the high speed-downlink shared channel (HS-DSCH) employed in third generation, wideband code division multiple access (WCDMA) systems, offer high data transmission rates, reduced round-trip delays, and high capacity, at least with respect to a typical transport channel. The HS-DSCH supports higher order modulation to achieve higher user and system throughput, fast link adaptation that takes into account the instantaneous quality of the radio propagation environment and adapts the coding and modulation scheme accordingly, and hybrid-ARQ with soft combining to decrease retransmissions and thereby reduce delay.
HS-DSCH mobile radio users periodically measure the instantaneous radio channel quality of a pilot channel broadcast by a radio base station, which is called Node B in the WCDMA specification. The mobile users periodically report a channel quality indicator (CQI) based upon the measured radio channel transmission. The base station responsible for handling the HS-DSCH uses the CQI to assign an appropriate coding and modulation scheme. It may also use the CQI to decide which mobile radio should be scheduled to receive downlink transmission over the HS-DSCH. Various scheduling strategies can be used for transmitting over the high speed shared channel.
There is an inherent time delay between the time instant when a mobile user reports the CQI and the time instant that the base station schedules transmission over the high speed shared channel to a mobile user. During this time delay, the interference may change dramatically for reasons described below. If the difference between the reported channel quality and the actual channel quality at the time of scheduling is large, the selected coding and modulation scheme may not be sufficiently robust to ensure transmission with a low enough error rate. If the data is received in error, the mobile radio requests retransmission which degrades system performance.
This difference between a reported CQI and the actual CQI at the scheduled HS-DSCH transmission is particularly problematic in adaptive antenna systems. An adaptive antenna system can change its beam characteristics in response to changes in the network. An antenna beam is any signal transmission deliberately covering only part of a cell. A cell is a coverage area of the base station. Because the base station can detect the direction of a mobile station, it can transmit dedicated information in an antenna beam towards the desired mobile station. By directing the signal just toward its recipient, the interference in the network can be substantially reduced. Adaptive antennas can significantly increase the data capacity in a cellular radio network.
The discrepancy between the reported channel quality and the instantaneous channel quality caused by scheduling different mobile users to receive transmissions over a shared radio channel may be traced in large part to a “flashlight effect.” The flashlight effect will be described in conjunction with FIGS. 1, 2, 3, and 4. FIG. 1 illustrates a base station with three cells or sectors. In the upper, left-hand sector cell, the base station transmits a sector antenna beam which covers most of that sector cell. An adaptive antenna array in the right-most sector cell transmits five, relatively narrow antenna beams 1-5. Most antenna patterns contain a main lobe and several minor lobes commonly known as side lobes. The term “beam” refers to the main lobe. Eight mobile radios, (a mobile radio is referred to in WCDMA as a user equipment (UE)), are shown in or close to the right-most sector cell and are identified as U1-U8.
FIG. 2 illustrates an example situation where mobile radio U3 is scheduled for a current time instant to receive information over the high-speed downlink shared channel with maximum power. This is illustrated by the main and side lobes of beam B2 being represented in a bold solid line, with the remaining four beams B1, B3, B4, B5 carrying little or no power during this scheduled time period. By the end of the scheduled time period, all mobile radios, including the scheduled mobile radio U3, report to the base station their current or instantaneous detected channel quality indicator (CQI) based on the quality of reception of the base station's pilot signal. The base station transmits information to the next-scheduled mobile radio over the HS-DSCH channel at maximum power during the next scheduling time period.
In the example shown in FIG. 3, the next-scheduled mobile radio is U5 because at the time the scheduling decision was made, U5 had the highest CQI. Beam 3 B3, which encompasses mobile U5, is selected based on beam quality information detected at the base station. At the time instant shown FIG. 2, mobile radio U5 is at a “null” between the main lobe of beam B2 and a side lobe of beam B2. That null means U5 experiences low interference from the beam B2 transmission. On the other hand, mobile radio U4, which is relatively close to mobile radio U5, reports a much lower CQI because the main lobe of beam B2 creates a high interference at U4 on the order of 15 dB. Since mobile U4 is relatively close to the scheduled mobile U3, it is “blasted” by beam B2 and therefore reports a dramatically lower CQI than mobile U5. Yet, in the absence of the beam B2, the reported CQI from both mobiles U4 and U5 would be approximately the same. These kinds of “blasts” cause the flashlight effect.
The flashlight effect can be further illustrated using the example transmission scheduling table shown in FIG. 4. Here six mobiles U1-U6 report detected channel quality indicators (CQI) for each transmission time interval (TTI) TTI1-TTI7. The highest CQI for each TTI is underlined, and the scheduled mobile user for each TTI is circled. In this example, it takes two TTIs for the mobile user with the highest CQI to be scheduled for transmission over the HS-DSCH channel.
The table illustrates that the scheduled mobile radio does not always have the highest CQI during its scheduled TTI. For example, mobile U1 is scheduled to receive a transmission during TTI 5. Mobile U2, which is served by the same beam B1 as mobile U1, reports a very low CQI of 5 during TTI 5 because it is being temporarily “blasted” by the beam B1 transmission to mobile user U1. As a result, mobile U6 reports the highest CQI and is scheduled for TTI 7. Absent the flashlight effect of transmitting to mobile U1 during TTI 5, mobile U2 would have reported a much higher CQI. Indeed, after the flashlight effect of the beam B1 transmission during TTI's 5 and 6 subsides, mobile U2 reports a channel quality of 20, which is higher than the CQI of 18 reported by the scheduled mobile U6. The rapid and dramatic CQI increase from 4 to 20 for mobile U2 between TTI 6 and TTI 7 demonstrates the flashlight effect of beam B1 on mobile U2. This dramatic and rapid change of reported CQI from one scheduled time interval to the next is the flashlight effect.
In summary, the flashlight effect is intense interference detected by a mobile causing that mobile to report a low CQI for a short time period which results from the mobile being “flashed” by a brief downlink transmission to another scheduled mobile. The flashlight effect is a serious problem in fixed multi-beam systems, adaptive antenna systems, and transmit diversity systems.
The flashlight effect is overcome by selecting multiple mobile radios to receive a transmission over a shared radio channel during a predetermined transmission time interval. Information is transmitted over the shared radio channel to the multiple mobile radios using multiple antenna beams so that interference from the transmission appears as white additive Gaussian noise in time and in space in the cell. The “flashlight effect” caused by a single beam transmission over the shared channel that would detrimentally impact a mobile radio's detection of channel quality is avoided.
Mobile radios detect the channel quality of a pilot or other broadcast signal transmitted in the cell and report to the radio network. Shared channel transmissions are scheduled to multiple mobile radios for each time interval based on the received reports. One mobile radio is selected for transmission for each antenna beam based on the received reports. The information is transmitted over the shared channel using each antenna beam to each of the selected mobile radios during the predetermined time interval.
The shared radio channel radio resources are allocated to the multiple mobile radios using a resource allocation scheme. An optimal coding and modulation scheme is preferably selected for each scheduled mobile radio to achieve an acceptable error rate. Example resource allocation schemes include dividing the shared radio channel resources evenly between each selected mobile radio. The resource allocation scheme may divide the shared radio channel resources in proportion to each of the mobile radio's reported detected channel quality, e.g., in accordance with a “water pouring” distribution algorithm. Alternatively, the shared channel resources may be divided using a non-linear relationship between two or more of the following: amount of channel resources, throughput, quality of service, and detected channel quality. That relationship may be stored in a lookup table for easy application. If a change is detected in radio channel conditions, the look-up table is preferably updated.
In the situation where the radio communications system is a CDMA-based system and radio channel resources include scrambling codes, with each scrambling code having an associated channelization code tree, an example resource allocation scheme includes allocating one scrambling code to the shared radio channel. One or more different channelization codes associated with that scrambling code are allocated to each antenna beam during each predetermined time interval. Alternatively, a different scrambling code may be allocated for each antenna beam during each predetermined time interval.
The flashlight effect may be avoided by carefully planning in space and/or in time which beam is used for transmission. Another technique for avoiding the flashlight effect employs a beam transmission sequence order. Multiple mobile radios may be selected to receive a transmission over a shared radio channel using a beam transmission sequence order. Mobile users belonging to a selected beam may be scheduled. The beam selection is decided using a beam sequence number. Information is transmitted over the shared radio channel to each of the mobile radios in the cell following the beam transmission sequence order. Beam switching in accordance with the beam transmission sequence order occurs over multiple transmission time intervals so that interference from the transmission appears as white noise in time and in space.